Technologies on the optical memory concerned with the optical disk with large capacity and high density have come to be used for various purposes, such as digital audio and video disks and data files, and the digital video disk with large capacity and high density being seven times as much as those of the conventional one is put to practical use. In the technology on the optical memory, since information is recorded and reproduced through a laser beam having a spot size in the order of micro meter with high reliability, it is very important to develop the optical head apparatus with high quality and excellent performance.
Fundamental functions of the optical head apparatus realized by the intensive optical technology are classified into a function of a spot-focussing to the limit of diffraction, that of focus-controlling of the spot, that of pit signal-detecting by the spot and that of track-controlling of the spot.
These functions are actualized by combining various optical instruments and optical to electrical converters in accordance with their purposes and their usages.
Recently, high density optical disk systems employing 0.6 mm substrate thickness, such as Digital Video Disk (DVD, hereinafter) have been standardized. In these systems, compatibility with conventional optical disk systems employing 1.2 mm substrate thickness, such Compact Disk (CD, hereinafter), is strongly required. In case of reading a 0.6 mm disk and a 1.2 mm disk using the same optical head apparatus, it is necessary to compensate spherical aberration due to the difference in the substrate thickness. Several kinds of optical head apparatus have been proposed for the same purpose, such as a twin lens type, a variable aperture type and a dual focus type. However, the optical head apparatus of the aforementioned types comprise complicated mechanisms for adjusting focuses, are weighty and high-priced and deteriorate aberration characteristic, performance of recorded signal-reproducing characteristic, access speed and bit rate of data translation.